Tri wavelength diffraction modulator and a method for modulation

ABSTRACT

The present invention relates to a tri wavelength diffraction modulator (TWDM) and a method of tri wavelength diffraction modulation. The tri wavelength diffraction modulator includes: a stationary substrate with a bottom electrode plate formed on top of the stationary substrate; a first electrode plate comprising a first suspended beam suspended in parallel above the stationary substrate and a first connection anchored onto the stationary substrate; and a second electrode plate comprising a second suspended beam suspended in parallel above the first electrode plate and a second connection anchored onto the stationary substrate. The diffraction modulator and the method for diffraction modulation are suitable to projection system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No.PCT/CN2010/080643, filed on Dec. 31, 2010, which claims the prioritybenefit of American Application No. 61/292,107, filed on Jan. 4, 2010.The contents of the above identified applications are incorporatedherein by reference in their entirety.

FIELD OF THE TECHNOLOGY

The present invention relates to a tri wavelength diffraction modulator(TWDM) and a method of tri wavelength diffraction modulation, whichbelongs to the technology field of optical modulation device.

BACKGROUND

Optional modulators are solid state micro devices, which are widely usedfor microdisplay projection and other optical signal processing systems.Optional modulators can control or modulate an incident beam of light ina defined spatial pattern. The defined spatial pattern is correlated toa series of electrical inputs of image to the devices, through which theincident light beam can be modulated in intensity, phase, polarizationor direction.

Two of the most commonly used classes of optical and particularly,spatial light modulators employ microelectromechanical system (MEMS)devices in a two dimensional array configured to provide two-dimensionalmodulation of incident light: Digital Micromirror Device (DMD) fromTexas Instruments and the Grating Light Valve (GLV) device from SiliconLight Machines.

The appeal of the DMD has been evidenced in the widespread adoption,given its high optical efficiency, large etendue, wide bandwidth, highmodulation speed and digitalized control algorithm for time sequentialcolor combination and management. Despite its success in projectiondisplay applications, however, the DMD has been recognized with certainshortcomings, such as high power consumption per pixel, particularly forhigh resolution microdisplay projection applications in cellphone andhandheld devices.

The GLV array devices based in fine reflective metal grids or elementsare also recognized with significant appeal in etendue, analoggrey-scaling, optical efficiency, modulation speed and particularly,power consumption per pixel. In a either linear or 2 dimensionalconfiguration, a GLV array for modulating incident beams of light, themodulator comprising a plurality of grating elements, each of whichincludes a light reflective planar surface. Those grating elements arearranged parallel to each other with their light reflective surfacesparallel to each other. The modulator includes electrical-mechanicalmeans for supporting the elements in relation to one another and meansfor moving the elements relative to one another so that elements movebetween a first configuration wherein the modulator acts to reflect theincident beam of light as a plane mirror, and a second configurationwherein the modulator diffracts the incident beam of light as it isreflected therefrom. In operation, the light reflective surfaces of theelements remain parallel to each other in both the first and the secondconfigurations and the perpendicular spacing between the reflectivesurfaces of adjacent elements is equal to m/4 times the wavelength ofthe incident rays of light, wherein m is an even whole number or zerowhen the elements are in the first configuration and m is an odd numberwhen the beam elements are in the second configuration.

The core idea of such a GLV modulator includes a reflective deformablegrating light modulator, with a grating amplitude that can be controlledelectronically, consisting of a reflective substrate with a deformablegrating suspended above it. In its undeformed state, with no voltageapplied between the elements of the grating and the substrate, thegrating amplitude is one half of the wavelength of the incoming light.Since the round-trip path difference between the light reflected fromthe top and bottom of the grating is one wavelength, no diffractionoccurs. When a voltage is applied between the grating elements and thesubstrate, the electrostatic force pulls the elements down to cause thegrating amplitude to become one quarter of the wavelength so thatreflections from the elements and the substrate add destructively,causing the light to be diffracted. If the detection system for thereflected light has a numerical aperture which accepts only the zeroorder beam, a mechanical motion of only one quarter of a wavelength issufficient to modulate the reflected light with high contrast.

However, the wavelength dependency under a control algorithm withdiscrete states of light modulation and incident angle sensitivity dueto diffraction are evident on the GLV devices disclosed in the priorart. Particularly for microdisplay projection applications, digitalizedspatial modulation is desired for modulating illumination of wideincident angle over visible spectrum and in particular, in associationwith time sequential or spatial mosaic pattern color management schemes.

SUMMARY

The present invention provides a tri wavelength diffraction modulatorand method of tri wavelength diffraction modulation, so that themodulating process is suitable to a projection system.

One aspect of the present invention provides a tri wavelengthdiffraction modulator, including:

-   -   a stationary substrate with a bottom electrode plate formed on        top of the stationary substrate;    -   a first electrode plate comprising a first suspended beam        suspended in parallel above the stationary substrate and a first        connection anchored onto the stationary substrate; and    -   a second electrode plate comprising a second suspended beam        suspended in parallel above the first electrode plate and a        second connection anchored onto the stationary substrate;    -   wherein the second suspended beam of the second electrode plate        comprises a second reflecting layer and a second dielectric        layer, at least one micro aperture is opened in the second        reflecting layer;    -   the stationary substrate further comprises a driving circuitry        built inside the stationary substrate, the driving circuitry is        adapted to provide electrical charge to the bottom electrode,        the first electrode plate and the second electrode plate        respectively, so as to remain a relax distance, a pull-close        distance and a pull-apart distance between the first suspended        beam and the second suspended beam respectively within different        time durations;    -   the pull-close distance is adapted to make illumination of a        first wavelength in incident illumination passing through the        micro aperture to form diffraction, the relax distance is        adapted to make illumination of a second wavelength in incident        illumination form diffraction, and the pull-apart distance is        adapted to make illumination of a third wavelength in incident        illumination form diffraction.

Another aspect of the present invention provides a method of triwavelength diffraction modulation, including:

-   -   dividing the first duration into a first off duration and a        first on duration;    -   driving the first electrode plate and the second electrode plate        by the driving circuitry of the tri wavelength diffraction        modulator to form a relative movement, and remain a relax        distance between the first suspended beam and the second        suspended beam during the second off duration so that the        illumination of the second wavelength forms diffraction, but not        remain the relax distance between the first suspended beam and        the second suspended beam so that the illumination of the second        wavelength forms reflection during reflection during the first        on duration;    -   dividing the second duration into a second off duration and a        second on duration;    -   driving the first electrode plate and the second electrode plate        by the driving circuitry of the tri wavelength diffraction        modulator to form a relative movement, and remain a relax        distance between the first suspended beam and the second        suspended beam during the second off duration so that the        illumination of the second wavelength forms diffraction, but not        remain the relax distance between the first suspended beam and        the second suspended beam so that the illumination of the second        wavelength forms reflection during the second on duration;    -   dividing the third duration into a third off duration and a        third on duration;    -   driving the first electrode plate and the second electrode plate        by the driving circuitry of the tri wavelength diffraction        modulator to form a relative movement, and remain a pull-apart        distance between the first suspended beam and the second        suspended beam during the second off duration so that the        illumination of the third wavelength forms diffraction, but not        remain the pull-apart distance between the first suspended beam        and the second suspended beam so that the illumination of the        third wavelength forms reflection during the third on duration.

The diffraction modulator and the method for diffraction modulation inthe present invention described above achieve the pulse width modulation(PWM) to the incident illumination by using diffraction, which benefitsthe integration of digitalized control algorithm and modulated compositewave, and achieves effective modulation for illumination of wideincident angle. Therefore, the diffraction modulator and the method fordiffraction modulation are suitable to projection system.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings are not necessarily to scale, emphasis instead being placedupon illustrating the framework and principles of the disclosedinvention.

FIG. 1 a is a cross sectional view of a tri wavelength diffractionmodulator in one embodiment of the present invention, illustrating afirst suspended beam remains a relax distance 22 to a second suspendedbeam.

FIG. 1 b is a cross sectional view of a tri wavelength diffractionmodulator in one embodiment of the present invention, illustrating afirst suspended beam remains a pull-apart distance 23 to a secondsuspended beam.

FIG. 1 c is a cross sectional view of a tri wavelength diffractionmodulator in one embodiment of the present invention, illustrating afirst suspended beam remains a pull-close distance 21 to a secondsuspended beam.

FIG. 2 a is a schematic diagram showing a method of tri wavelengthdiffraction modulation in one embodiment of the present invention,illustrating the distribution of illustration intensity beforemodulating incident illumination 10.

FIG. 2 b is a schematic diagram showing a method of tri wavelengthdiffraction modulation in one embodiment of the present invention,illustrating the relations of the distance between a first suspendedbeam and a second suspended beam in the modulating process and theillumination intensity.

DETAILED DESCRIPTION

In order to make the objects, technical solutions and merits of thepresent invention clearer, a further detailed description of embodimentsof the present invention is given by reference to accompanying drawings.Furthermore, for purposes of clarity, part of the extended detail ofthose novel devices, that are widely known and are not relevant to thepresent invention, have been omitted from the following description.

As shown in FIGS. 1 a-1 c, the tri wavelength diffraction modulator(TWDM) in this embodiment comprises a stationary substrate 200, a firstelectrode plate 110 and a second electrode plate 120.

A bottom electrode plate 210 is formed on a top of the stationarysubstrate 200; the first electrode plate 110 comprises a first suspendedbeam suspended in parallel to and above the stationary substrate 200 anda first connection anchored onto the stationary substrate 200; thesecond electrode plate 120 comprises a second suspended beam suspendedin parallel above the stationary substrate 200 and a second connectionanchored onto the stationary substrate 200.

The second suspended beam of the second electrode plate 120 comprises asecond reflecting layer 125 and a second dielectric layer 126. At leastone micro aperture 125 a is opened in the second reflecting layer 125.The micro aperture 125 a may be shaped in any close-loop geometricfigure, such as circle, ring, ellipse, or polygon. The second electrode120 can transmit incident illustration 10. The stationary substrate 200further comprises a driving circuitry 220, the driving circuitry 220 isbuilt inside the stationary substrate 200 and provides electrical chargeindividually to the bottom electrode 210, the first electrode plate 110and the second electrode plate 120, so as to keep a relax distance 22, apull-close distance 21, and a pull-apart distance 23 between the firstsuspended beam and the second suspended beam respectively withindifferent time durations.

Specifically, as is shown in FIG. 1 a, when electrical charge providedby the driving circuitry 220 is zero, the first suspended beam of thefirst electrode plate 110 remains a relax distance 22 to the secondsuspended beam of the second electrode plate 120. In addition,optionally, the relax distance 22 between the first suspended beam andthe second suspended beam can also be kept when the electrostatic forcescaused by electrical charge on the bottom electrode plate 210, the firstelectrode plate 110 and the second electrode plate 120 remainsequilibrium.

As is shown in FIG. 1 b, when the driving circuitry 220 providesopposite electrical charge to the first electrode plate 110 and thebottom electrode plate 210, the first electrode plate 110 moves to thebottom electrode plate 210 along a departing direction 51. A firstspacing limiter 131 is configured on a bottom of the first electrodeplate 110 protruding toward the bottom electrode plate 210, or on a topof the bottom electrode plate 210 protruding toward the first electrodeplate 110. When the first electrode plate 110 moves to the bottomelectrode plate 210, the first suspended beam of the first electrodeplate 110 keeps the pull-apart distance 23 from the second suspendedbeam of the second electrode plate 120 by the spacing limitation of thefirst spacing limiter 131.

Optionally, the first spacing limiter 131 is mounted on a bottom of thefirst electrode plate 110 or is integrated with the first electrodeplate 110, or is configured directly on a top of the bottom electrodeplate 210.

As is shown in FIG. 1 c, when the driving circuitry 220 providesopposite electrical charge to the first electrode plate 110 and thebottom electrode plate 210, the first electrode moves to the secondelectrode plate 120 along a closing direction 52. A second spacinglimiter 132 is configured on a bottom of the second electrode plate 120or a top of the first electrode plate 110. When the first electrodeplate 110 moves to the second electrode plate 120, the first suspendedbeam of the first electrode plate 110 keeps the pull-close distance 21from the second suspended beam of the second electrode plate 120 by thespacing limitation of the second spacing limiter 132.

Optionally, the second spacing limiter 132 is mounted on a bottom of thesecond electrode plate 120 or is integrated with the second electrodeplate 120, or is configured directly on a top of the first electrodeplate 110.

For achieving desired grey-scale control in a binary mode of pulse widthmodulation (PWM), the first suspended beam of the first electrode plate110 keeps a relax distance 22, a pull-close distance 21, or a pull-apartdistance 23 from the second suspended beam of the second electrode plate120 respectively within different time durations. The pull-closedistance 21 is used for making illumination of a first wavelength 91 inincident illumination 10 passing through the micro aperture 125 a toform diffraction, the relax distance 22 is used for making illuminationof a second wavelength 92 in incident illumination 10 form diffraction,and the pull-apart distance 23 is used for making illumination of athird wavelength 93 in incident illumination 10 form diffraction.

On the visible spectrum, the first wavelength 91 preferably correspondsto a chosen blue wavelength from 450 to 495 nm, the second wavelength 92preferably corresponds to a chosen green wavelength from 495 to 570 nm,and the third wavelength preferably corresponds to a chosen redwavelength from 620 to 750 nm.

Optionally, the bottom electrode plate 210 is made from any orcombination of silver, aluminum, copper, titanium, platinum, gold,nickel and cobalt, or other metal material.

Optionally, the first suspended beam further comprises a firstdielectric layer 116 and a first reflecting layer 115 formed on a top ofthe first dielectric layer 116, the first reflecting layer 115 is usedfor reflecting illumination that does not form diffraction andtransmitting the illumination out of the second electrode plate 120. Thefirst dielectric layer 16 and the first reflecting layer 115 can form acomposite plate. The first dielectric layer 116 is made from any one orcombination of silicon oxide, nitride and carbide, the first reflectinglayer 115 is made from any or combination of silver, aluminum, copper,titanium, platinum, gold, nickel and cobalt.

In order to obtain the maximum contrast ratio when modulatingdiffraction, the size of the micro aperture 125 a can be configured sothat the reflectivity of a area on the first reflecting layer 115corresponding to the micro aperture 125 a is equal to the reflectivityof a area on the second reflecting layer 125 except the micro aperture125 a.

Optionally, the second suspended beam of the second electrode plate 120further comprises a second reflecting layer 125 and a second dielectriclayer 126. The second dielectric layer 126 is made from any one orcombination of silicon oxide, nitride and carbide, the second reflectinglayer 125 is made from any or combination of silver, aluminum, copper,titanium, platinum, gold, nickel and cobalt.

A method of tri wavelength diffraction modulation in some embodiments ofthe present invention will be introduced in the following description.

As is shown in FIG. 2 a, the incident illumination 10 in the presentembodiment consists of illumination of the first wavelength 91 lastingfor a first duration 11, illumination of the second wavelength 92lasting for a second duration 12, and illumination of the thirdwavelength 93 lasting for a third duration 13. As is shown in FIG. 2 a,the illumination of the first wavelength 91, the illumination of thesecond wavelength 92, and the illumination of the third wavelength 93can have different illumination intensity.

As is shown in FIG. 2 b, the method comprises the following steps:

Step 111, dividing the first duration 11 into a first off duration 11 fand a first on duration 11 n;

Step 112, driving the first electrode plate 110 and the second electrodeplate 120 by a driving circuitry 220 of the tri wavelength diffractionmodulator 100 to form a relative movement, and remain a pull-closedistance 21 between a first suspended beam of the first electrode plate110 and a second suspended beam of the second electrode plate 120 duringthe first off duration 11 f so that the illumination of the firstwavelength 91 forms diffraction, but not remain the pull-close distance21 between the first suspended beam and the second suspended beam sothat the illumination of the first wavelength 91 forms reflection duringthe first on duration 11 n.

Specifically, the illumination of the first wavelength 91 is reflectedby the first reflecting layer 115 of a top of the first electrode plate110 and transmits out of the second electrode plate 120.

Step 121, dividing the second duration 12 into a second off duration 12f and a second on duration 12 n;

Step 122, driving the first electrode plate 110 and the second electrodeplate 120 by the driving circuitry 220 of the tri wavelength diffractionmodulator 100 to form a relative movement, and remain a relax distance22 between the first suspended beam and the second suspended beam duringthe second off duration 12 f so that the illumination of the secondwavelength 92 forms diffraction, but not remain the relax distance 22between the first suspended beam and the second suspended beam so thatthe illumination of the second wavelength 92 forms reflection during thesecond on duration 12 n.

Specifically, the illumination of the second wavelength 92 is reflectedby the first reflecting layer 115 of a top of the first electrode plate110 and transmits out of the second electrode plate 120.

Step 131, dividing the third duration 13 into a third off duration 13 fand a third on duration 13 n;

Step 132, driving the first electrode plate 110 and the second electrodeplate 120 by the driving circuitry 220 of the tri wavelength diffractionmodulator 100 to form a relative movement, and remain a pull-apartdistance 23 between the first suspended beam and the second suspendedbeam during the second off duration 13 f so that the illumination of thethird wavelength 93 forms diffraction, but not remain the pull-apartdistance 23 between the first suspended beam and the second suspendedbeam so that the illumination of the third wavelength 93 formsreflection during the third on duration 13 n.

Specifically, the illumination of the third wavelength 93 is reflectedby the first reflecting layer 115 of a top of the first electrode plate110 and transmits out of the second electrode plate 120. The firstreflecting layer 115 and the second reflecting layer 125 of the presentembodiments have a switching function between reflection anddiffraction.

The diffraction modulator and the method for diffraction modulation inthe embodiments of the present invention described above achieve thepulse width modulation (PWM) to the incident illumination 10 by usingdiffraction, which benefits the integration of digitalized controlalgorithm and modulated composite wave, and achieves effectivemodulation for illumination of wide incident angle. Therefore, thediffraction modulator and the method for diffraction modulation aresuitable to projection system.

The method for fabricating such a diffraction modulator incorporates astate-of-art silicon-based thin film processing technology used formaking modem MEMS devices in a multi-layer membrane configuration. Thebasic scheme of such fabrication employs forming a multiple membraneswith specific spacing between thereof suspended above a planarsubstrate, as shown in FIG. 1 a. Such fabrication involves firstdepositing and lithographically patterning a sacrificial thin film, thendepositing and lithographically patterning a structural thin film as themembrane, and lastly but not finally, selectively removing thesacrificial thin film but leaving the membrane intact suspended abovethe substrate. By repeating the same scheme, such a modulator devicewith a multiple membranes suspended and spaced in well-defined spacingis fabricated similar to the skill disclosed in prior art in the area ofthin film MEMS fabrication.

The present disclosure should not be considered limited to theparticular examples described above, but rather should be understood tocover all aspects of the invention as fairly set out in the attachedclaims. Various modifications, equivalent processes, as well as numerousstructures to which the present disclosure may be applicable will bereadily apparent to those of skill in the art to which the presentdisclosure is directed upon review of the instant specification.

1. A tri wavelength diffraction modulator, comprising: a stationarysubstrate with a bottom electrode plate formed on top of the stationarysubstrate; a first electrode plate comprising a first suspended beamsuspended in parallel above the stationary substrate and a firstconnection anchored onto the stationary substrate; and a secondelectrode plate comprising a second suspended beam suspended in parallelabove the first electrode plate and a second connection anchored ontothe stationary substrate, wherein the second suspended beam of thesecond electrode plate comprises a second reflecting layer and a seconddielectric layer, at least one micro aperture is opened in the secondreflecting layer; the stationary substrate further comprises a drivingcircuitry built inside the stationary substrate, the driving circuitryis adapted to provide electrical charge to the bottom electrode, thefirst electrode plate and the second electrode plate respectively, so asto remain a relax distance, a pull-close distance and a pull-apartdistance between the first suspended beam and the second suspended beamrespectively within different time durations; the pull-close distance isadapted to make illumination of a first wavelength in incidentillumination passing through the micro aperture form diffraction, therelax distance is adapted to make illumination of a second wavelength inincident illumination form diffraction, and the pull-apart distance isadapted to make illumination of a third wavelength in incidentillumination form diffraction.
 2. The tri wavelength diffractionmodulator according to claim 1, wherein the relax distance between thefirst suspended beam of the first electrode plate and the secondsuspended beam of the second electrode plate is remained when theelectrical charge provided by the driving circuitry is zero, or theelectrical charge remains equilibrium of electrostatic force among thebottom electrode plate, the first electrode plate and the secondelectrode plate.
 3. The tri wavelength diffraction modulator accordingto claim 1, wherein the first electrode is adapted to move towards thesecond electrode when the driving circuitry provides opposite electricalcharge to the first electrode plate and the second electrode plate. 4.The tri wavelength diffraction modulator according to claim 3, wherein asecond spacing limiter is configured on a bottom of the second electrodeplate or a top of the first electrode plate, the second spacing limiteris adapted to remain the pull-close distance between the first suspendedbeam of the first electrode plate and the second suspended beam of thesecond electrode plate when the first electrode plate is moving towardsthe second electrode plate.
 5. The tri wavelength diffraction modulatoraccording to claim 1, wherein the first electrode is adapted to movetowards the bottom electrode plate when the driving circuitry providesopposite electrical charge to the first electrode plate and the bottomelectrode plate.
 6. The tri wavelength diffraction modulator accordingto claim 5, wherein a first spacing limiter is configured on a bottom ofthe first electrode plate or a top of the bottom electrode plate, thefirst spacing limiter is adapted to remain the pull-apart distancebetween the first suspended beam of the first electrode plate and thesecond suspended beam of the second electrode plate when the firstelectrode plate is moving towards the bottom electrode plate.
 7. The triwavelength diffraction modulator according to claim 1, wherein the firstwavelength, the second wavelength and the second wavelength correspondto a chosen blue wavelength within blue color spectrum from 450 to 495nm, a chosen green wavelength within green color spectrum from 495 to570 nm, and a chosen red wavelength within red spectrum from 620 to 750nm, respectively.
 8. The tri wavelength diffraction modulator accordingto claim 1, wherein the bottom electrode plate is made from any orcombination of silver, aluminum, copper, titanium, platinum, gold,nickel and cobalt.
 9. The tri wavelength diffraction modulator accordingto claim 1, wherein the first suspended beam further comprises a firstdielectric layer and a first reflecting layer formed on a top of thefirst dielectric layer, the first reflecting layer is adapted to reflectand transmit illumination that does not form diffraction in the incidentillumination out of the second electrode plate.
 10. The tri wavelengthdiffraction modulator according to claim 10, wherein the firstdielectric layer is made from any one or combination of silicon oxide,nitride and carbide, the first reflecting layer is made from any orcombination of silver, aluminum, copper, titanium, platinum, gold,nickel and cobalt.
 11. The tri wavelength diffraction modulatoraccording to claim 1, wherein the micro aperture is shaped in aclose-loop geometric figure.
 12. The tri wavelength diffreactionmodulator according to claim 11, wherein the close-loop geometric figureis circle, ring, ellipse, or polygon.
 13. The tri wavelength diffractionmodulator according to claim 1, wherein the second dielectric layer ismade from any one or combination of silicon oxide, nitride and carbide,the second reflecting layer is made from any or combination of silver,aluminum, copper, titanium, platinum, gold, nickel and cobalt.
 14. Amethod of tri wavelength diffraction modulation by using the triwavelength diffraction modulator according to claim 1 to modulate theincident illumination, wherein the incident illumination consists ofillumination of the first wavelength lasting for a first duration,illumination of the second wavelength lasting for a second duration, andillumination of the third wavelength lasting for a third duration, themethod comprising: dividing the first duration into a first off durationand a first on duration; driving the first electrode plate and thesecond electrode plate by a driving circuitry of the tri wavelengthdiffraction modulator to form a relative movement, and remain apull-close distance between a first suspended beam of the firstelectrode plate and a second suspended beam of the second electrodeplate during the first off duration so that the illumination of thefirst wavelength forms diffraction, but not remain the pull-closedistance between the first suspended beam and the second suspended beamso that the illumination of the first wavelength forms reflection duringthe first on duration; dividing the second duration into a second offduration and a second on duration; driving the first electrode plate andthe second electrode plate by the driving circuitry of the triwavelength diffraction modulator to form a relative movement, and remaina relax distance between the first suspended beam and the secondsuspended beam during the second off duration so that the illuminationof the second wavelength forms diffraction, but not remain the relaxdistance between the first suspended beam and the second suspended beamso that the illumination of the second wavelength forms reflectionduring the second on duration; dividing the third duration into a thirdoff duration and a third on duration; driving the first electrode plateand the second electrode plate by the driving circuitry of the triwavelength diffraction modulator to form a relative movement, and remaina pull-apart distance between the first suspended beam and the secondsuspended beam during the second off duration so that the illuminationof the third wavelength forms diffraction, but not remain the pull-apartdistance between the first suspended beam and the second suspended beamso that the illumination of the third wavelength forms reflection duringthe third on duration.